One of the things about the rod weight difference is to consider what you are going to use it for.

I own from a 4 weight to an 8 in various makes and lengths and each is for a specific purpose.
The 4 weight is for small to medium rivers, the 5 weight for medium rivers, the 6 weight is for dry fly work on the loughs here in Ireland while I use a 7 weight for wet work on the same waters. The 8 weight hardly sees any action at all these days and I have a 12ft 5-7 weight boat rod that I used to use exclusively on the Scottish lochs that is gathering dust as my casting style has changed.

Rounding it up, it is a case of horses for courses and in saying that, personal preference is still the major factor in deciding what weight of rod to use.